June 2018 p61 q5
3017
In Pelmerdon 22% of families own a dishwasher.
Find the probability that, of 15 families chosen at random from Pelmerdon, between 4 and 6 inclusive own a dishwasher.
Solution
Let the probability of a family owning a dishwasher be \(p = 0.22\). The number of families is \(n = 15\).
We need to find \(P(4 \, \text{to} \, 6)\), which is the probability that between 4 and 6 families own a dishwasher.
Using the binomial probability formula:
\(P(X = k) = \binom{n}{k} p^k (1-p)^{n-k}\)
Calculate for \(k = 4, 5, 6\):
\(P(4) = \binom{15}{4} (0.22)^4 (0.78)^{11}\)
\(P(5) = \binom{15}{5} (0.22)^5 (0.78)^{10}\)
\(P(6) = \binom{15}{6} (0.22)^6 (0.78)^9\)
Sum these probabilities:
\(P(4, 5, 6) = \binom{15}{4} (0.22)^4 (0.78)^{11} + \binom{15}{5} (0.22)^5 (0.78)^{10} + \binom{15}{6} (0.22)^6 (0.78)^9\)
\(= 0.398\)
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